Hemostasis, the arrest of bleeding from an injured blood vessel, requires the concerted activity of vascular, platelet, and plasma factors to eventually form a hemostatic seal or a blood clot. In normal hemostasis, the combined activity of these factors is counterbalanced by regulatory mechanisms to limit the accumulation of platelets and fibrin in the area of injury.
Upon injury to a blood vessel, vascular factors reduce blood flow from the blood vessel by local vasoconstriction and compression of injured vessels. At the same time, platelets adhere to the site of vessel wall injury and form aggregates called hemostatic plugs, which form the first key element of the hemostatic seal. Platelets also release factors that provide surface membrane sites and components for the formation of enzyme/cofactor complexes in blood coagulation reactions. Through a series of interacting and propagating zymogen activations, the activated form of one plasma factor catalyzes the activation of the next plasma factor. This cascade of blood coagulation reactions eventually forms a fibrin clot. The fibrin clot, an insoluble fibrin matrix that radiates from and anchors the hemostatic plug, is the second key element of the hemostatic seal
Specifically, the cascade of blood coagulation reactions discussed involvse two interdependent pathways, an intrinsic pathway and an extrinsic pathway. Each pathway ultimately catalyzes the proteolytic activation of factor X to factor Xa.
Damage to the blood vessel or a negatively charged surface initiates blood clotting by the intrinsic pathway. As seen in FIG. 1, the major components of the intrinsic pathway include factor VIII, a non-enzymatic co-factor, and factors IX and XI, zymogen serine proteases. The initiation of the intrinsic pathway results in the activation of factor XI to XIa. Factor XIa, as well as the presence of the factor VIIa/tissue factor complex involved in the extrinsic pathway, catalyzes the activation of factor IX to factor IXa. The presence of factor IXa, in combination with the activated form of factor VIII on an appropriate phospholipid surface, results in the formation of a tenase complex (10). The tenase complex catalyzes the formation of factor Xa from its zymogen, factor X.
Exposure of blood to injured tissue initiates blood clotting by the extrinsic pathway. As is shown in FIG. 1, the major components of the extrinsic pathway are factor VII, a zymogen serine protease, and tissue factor, a membrane bound protein. Tissue factor serves as the requisite non-enzymatic co-factor for factor VII. The initiation of the extrinsic pathway is thought to be an autocatalytic event resulting from the activation of factor VII by trace levels of activated factor VII (factor VIIa), both of which are bound to newly exposed tissue factor on membrane surfaces at sites of vascular damage (20). The factor VIIa/tissue factor complex directly catalyzes the formation of factor Xa from factor X.
Once the initial intrinsic or extrinsic cascade results in the activation of factor X, factor Xa catalyzes the penultimate step in the blood coagulation cascade, the formation of serine protease thrombin. As seen in FIG. 2, thrombin formation occurs when a prothrombinase complex, comprising of factor Xa, the non-enzymatic co-factor Va and the substrate prothrombin, is assembled on an appropriate phospholipid surface (30). Once formed, thrombin functions as part of a feedback loop, controlling the activation of factors V and VIII. It additionally catalyzes both the activation of factor VIII and the conversion of fibrinogen to fibrin. Finally, the factor VIIIa interacts with fibrin to catalyze the formation of a thrombus, or crosslinked fibrin clot.
In normal hemostasis, the process of clot formation (blood coagulation) and clot dissolution (fibrinolysis) is delicately balanced. A slight imbalance between the processes of clot formation and dissolution can lead to excessive bleeding or thrombosis. Many significant disease states are related to abnormal hemostasis. With respect to the coronary arterial vasculature, abnormal thrombus formation due to the rupture of an established atherosclerotic plaque is the major cause of acute myocardial infarction and unstable angina. Moreover, treatment of an occlusive coronary thrombus by either thrombolytic therapy or percutaneous transluminal coronary angioplasty (PTCA) is often accompanied by an acute thrombotic reclosure of the affected vessel which requires immediate resolution. With respect to the venous vasculature, a high percentage of patients undergoing major surgery in the lower extremities or the abdominal area suffer from thrombus formation in the venous vasculature which can result in reduced blood flow to the affected extremity and a predisposition to pulmonary embolism. Disseminated intravascular coagulopathy commonly occurs within both vascular systems during septic shock, certain viral infections and cancer and is characterized by the rapid consumption of coagulation factors and systemic coagulation which results in the formation of life-threatening thrombi occurring throughout the vasculature leading to widespread organ failure.
Pathogenic thrombosis in the arterial vasculature is a major clinical concern in today's medicine. It is the leading cause of acute myocardial infarction which is one of the leading causes of death in the western world. Recurrent arterial thrombosis also remains one of the leading causes of failure following enzymatic or mechanical recanalization of occluded coronary vessels using thrombolytic agents or percutaneous transluminal coronary angioplasty (PTCA), respectively [Ross, A. M., Thrombosis in Cardiovascular Disorder, p. 327, W.B. Saunders Co. (Fuster, V. and Verstraete, M. edit. 1991); Califf, R. M. and Willerson, J. T., Id. at p 389]. In contrast to thrombotic events in the venous vasculature, arterial thrombosis is the result of a complex interaction between fibrin formation resulting from the blood coagulation cascade and cellular components, particularly platelets, which make up a large percentage of arterial thrombi. Heparin, the most widely used clinical anticoagulant administered intravenously, has not been shown to be universally effective in the treatment or prevention of acute arterial thrombosis or rethrombosis [Prins, M. H. and Hirsh, J., J. Am. Coll. Cardiol., 67: 3A (1991)].
Besides the unpredictable, recurrent thrombotic reocclusion which commonly occurs following PTCA, a profound restenosis of the recanalized vessel occurs in 30 to 40% of patients 1 to 6 months following this procedure [Califf, R. M. et al., J. Am. Coll. Cardiol., 17: 2B (1991)]. These patients require further treatment with either a repeat PTCA or coronary artery bypass surgery to relieve the newly formed stenosis. Restenosis of a mechanically damaged vessel is not a thrombotic process but instead is the result of a hyperproliferative response in the surrounding smooth muscle cells which over time results in a decreased luminal diameter of the affected vessel due to increased muscle mass. Id. As for arterial thrombosis, there is currently no effective pharmacologic treatment for the prevention of vascular restenosis following mechanical recanalization.
Numerous strategies have been developed for the treatment of thrombotic disorders. Many antithrombotic therapies are based on interference in the hemostatic system. This approach carries the inherent risk of bleeding, since the hemostatic system is no longer fully responsive to potential injury. Therefore, antithrombotic benefits are normally associated with antihemostatic risks. In attempts to improve the benefit-to-risk ratio, antithrombotic agents are continuously being developed. Various antithrombotic strategies include administering general inhibitors of thrombin formation such as heparin or vitamin K antagonists; administering specific thrombin inhibitors; administering specific factor Xa inhibitors; and administering inhibitors of platelet activation and adhesion.
Evaluation of current antithrombotic strategies in terms of antithrombotic benefits versus antihemostatic risks reveals that the benefit-to-risk ratio tends to be more favorable for strategies that interfere with one specific step rather than in a more general phase of the hemostatic system [L. A. Harker, Biomedical Progress vol 8, 1995, 17–26]. For example, the development of inhibitors specific for factor Xa is an improvement from general and specific thrombin inhibitors. But, this approach still blocks the common (intrinsic and extrinsic) pathway of thrombin generation (see FIG. 1), and thereby thrombin-dependent platelet activation. Thus, a need exists for more specific antithrombotic agents that selectively inhibit one single hemostatic pathway, while leaving other pathways unaffected.